Encrypted message transmission in a QO-STBC encoded MISO wireless Communication system under implementation of low complexity ML decoding algorithm

In this paper, we made a comprehensive BER simulation study of a quasi-orthogonal space time block encoded (QO-STBC) multiple-input single output(MISO) system. The communication system under investigation has incorporated four digital modulations (QPSK, QAM, 16PSK and 16QAM) over an Additative White Gaussian Noise (AWGN) and Raleigh fading channels for three transmit and one receive antennas. In its FEC channel coding section, three schemes such as Cyclic, Reed-Solomon and ½-rated convolutionally encoding have been used. Under implementation of merely low complexity ML decoding based channel estimation and RSA cryptographic encoding /decoding algorithms, it is observable from conducted simulation test on encrypted text message transmission that the communication system with QAM digital modulation and ½-rated convolutionally encoding techniques is highly effective to combat inherent interferences under Raleigh fading and additive white Gaussian noise (AWGN) channels. It is also noticeable from the study that the retrieving performance of the communication system degrades with the lowering of the signal to noise ratio (SNR) and increasing in order of modulation .


INTRODUCTION
Data Security using cryptography has emerged as a topic of significant interest in both academic and industry circles. In data and telecommunications, cryptography is necessary when communicating over any untrusted network particularly Internet.
In recent year, a significant amount of research is being going on to enrich network security through development of Asymmetric key algorithm [1].
In asymmetric key cryptography, also called Public Key cryptography, two different keys (which forms a key pair) are used. One key is used for encryption and only the other corresponding key must be used for decryption. No other key can decrypt the message, not even the original (i.e. the first) key used for encryption. The beauty of this scheme is that every communicating party needs just a key pair for communicating with any number of other communicating parties. Once someone obtains a key pair, he /she can communicate with anyone else. RSA is a well-known public-key cryptography algorithm. It is the first algorithm known to be suitable for signing as well as encryption, and was one of the first great advances in public key cryptography [2] In wireless communications, one main challenge is the transmission over channels that experience time-variant multipath fading . Such detrimental effects in wireless fading channels can be combated using space -time block coding (STBC), an efficient transmit diversity scheme with exploitation of the diversity advantage of multi-antenna systems [3,4].Orthogonal space time block codes (OSTBCs) achieve full transmit diversity with a low (linear) maximum-likelihood (ML) decoding complexity . In case of multi-input multi-output (MIMO) communication systems with more than two transmit antennas, full-diversity rate-one O-STBCs do not exist. To overcome the low-rate constraints of OSTBCs, Quasi-orthogonal space time block codes (QO-STBCs) for four and three transmit antennas have been proposed which not achieve full diversity and have a substantially higher ML decoding complexity than the OSTBCs [5].In this present study , a low-complexity sub optimal ML decoder for coherent quasi-orthogonal space time block codes with three transmit antennas and RSA block cipher scheme based data encryption/decryption algorithm have been used. A brief description of these data processing schemes are outlined below.

RSA Algorithm
A cryptographic algorithm RSA (Rivest-Shamir-Adleman) was developed in 1977 by Ron Rivest, Adi Shamir, and Len Adleman .This RSA scheme is a block cipher in which the plaintext and ciphertext are integers between 0 and n 1 for n less than 21024. It makes use of an expression with exponentials .Plaintext is encrypted in blocks and each block size must be less than or equal to log2(n). In RSA, Encryption and Decryption are of the following form, for some plaintext block M and ciphertext block C:: C = M e mod n M = C d mod n = (M e ) d mod n = M ed mod n Both sender and receiver must know the value of n. The sender knows the value of e, and only the receiver knows the value of d. Thus, this is a public-key encryption algorithm with a public key of PU = {e, n} and a private key of PU = {d, n}. For this www.ijctonline.com algorithm to be satisfactory for public-key encryption, the following requirements must be met[ 10].

Low Complexity ML Decoding
In this section, the low-complexity sub optimal ML decoding scheme proposed by Samer J. Alabed and et.al in 2011 has been reviewed [5], In a MISO wireless communication system with three transmit and one receive antennas and assumed flat block fading channel with block length of T, the input-output relation can be expressed as: ……………………………. (1) and the QO-STBC for three transmitting antenna can be written to derive low-complexity decoding scheme as: where denotes the complex conjugate and are the transmitted digitally modulated signals that depend on The diversity order of two can be achieved if To achieve full diversity, the following mapping can be used [7].

The coherent QO-STBC
Using (4) and assuming the perfect CSI at the receiver, the coherent ML decoder of (2) can be written as + ………………………. (5) Where stand for the frobenious norm. Expanding the first and second term at the right hand side of (5). We obtain …(6) ……….. (7) Respectively, where and are constant terms that do not depend on the symbols and Denotes the real part. Using (6) and (7) in (5)

COMMUNICATION SYSTEM MODEL
A simulated single -user 3 x 1 spatially multiplexed wireless communication system as depicted in Figure 1 utilizes low complexity ML decoding scheme based Quasi-Orthogonal space-time block and o coding scheme. In such a communication system, the text message is converted into integer and subsequently encrypted using RSA encryption algorithm. The encrypted data are converted into binary bits and channel encoded using Cyclic, Reed-Solomon and ½-rated convolutionally encoding schemes and interleaved for minimization of burst errors. The interleaved bits are digitally modulated using various types of digital modulations such as Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude (QAM), 16PSK and 16QAM [8,9]. The complex digitally modulated symbols are block encoded with implemented QOSTBC scheme and fed into three transmitting antennas. In receiving section, the transmitted signal is processed with ML decoding algorithm and the decoded modulated symbols are fed into Q-OSTBC decoder. Its output data are demapped, deinterleaved and channel decoded . The decoded binary data are converted into integer and decrypted with RSA decryption algorithm.. The decrypted data are converted into message.

RESULTS AND DISCUSSION
We have made a comprehensive BER simulation study for a single-user MISO QO-STBC encoded wireless communication system based on the parameters given in Table 1. It is assumed that the channel state information (CSI) is available at the receiver and the fading process is approximately constant during each time slot assigned for simultaneous transmission of symbols from three transmitting antennas in a Q-OSTBC codeword. The graphical illustrations presented in Figure 2 through Figure 4 show system performance comparison with implementation of various channel coding and ML signal detection schemes under different digital modulations. In Figure  2, it is noticeable that for a typically assumed SNR value of 3 dB, the BER values are 0.0012 and 0.4562 in case of QAM and 16 PSK digital modulations viz., the system achieves a substantial gain of 25.80 dB in QAM as compared to 16PSK. In Figure 3, it is observable that the system shows comparatively better performance in convolutional encoding scheme as compared to cyclic coding. Under implementation of cyclic coding, the system shows almost flat response for wide range of SNR values. For a typically assumed SNR value of 3 dB, the BER values are 0.0012 and 0.3684 in case of convolutional and cyclic channel coding schemes viz., the system achieves a substantial gain of 24.87 dB in convolutional coding as compared to Cyclic. Figure 4 is clearly indicative of the impact of channel coding, The bit error rate in case of system performance with implemented channel and interleaving schemes and without implemented channel and interleaving schemes at a typically assumed low SNR value of 0.5 dB are 0.0178 and 0.0683 respectively. This is also a case of system performance improvement of 5.84 dB. Table 1: Summary of the simulated model parameters In Figure 5, the bit error rate at SNR value of 0dB and 5 dB are 0.0301 and 0.0007 respectively. The quality of transmitted encrypted message is improved with increase in SNR values. The use of multiple antennas at the transmitter and receiver in wireless systems, popularly known as MIMO (multiple-input multiple-output) technology, has rapidly gained in popularity over the past decade due to its powerful performance-enhancing capabilities. Communication in wireless channels is impaired predominantly by multi-path fading The use of multiple antennas at the transmitter and receiver in wireless systems, popularly known as MIMO (multiple-input multiple-output) technology, has rapidly gained in popularity over the past decade due to its poSerful performance-enhancing capabilities. Communicaton ·n wireless channels is impaired predominantly by multi-path fading ]

(c)
The use of multiple antennas at the transmitter and receiver in wireless systems, popularly known as MIMO (multiple-input multiple-output) technology, has rapidly gained in popularity over the past decade due to its powerful performance-enhancing capabilities. Communication in wireless channels is impaired predominantly by multi-path fading ]

Conclusions
In this paper, an effort has been made to present simulation results under implementation of various channel encoding techniques in a single -user low complexity ML decoding scheme based Quasi-Orthogonal space-time block encoded MISO wireless communication system. A range of system performance results highlights the impact of ML detection and channel coding scheme on encrypted text message transmission. In the